KABUL, Afghanistan (CNN) -- Two coalition U.S. soldiers were killed along with three Afghans, including a police official, while trying to disable a roadside bomb in southern Afghanistan Sunday, the U.S. military said.

Afghan police destroy poppy fields in Helmand province.

The explosion was under investigation, the military said.

The soldiers were part of a convoy of coalition troops accompanying Gulab Mangal, the governor of Helmand province, to a village where he intended to talk to residents about alternatives to opium farming.

The convoy came upon two bombs stacked on top of each other, said local journalist Abdul Tawab Qureshi.

When the soldiers tried to disable the bombs, the second one went off, added Qureshi, saying the blast killed the police chief of the province's Nad Ali district, Mohammed Nader; a police officer; and a translator.

Over the years, opium and heroin -- both derivatives of the poppy -- have served as a major source of revenue for the insurgency, most notably the Taliban movement that once ruled Afghanistan.

Though southern Afghanistan still provides about two-thirds of the world's opium and heroin, poppy cultivation has dropped by 20 percent -- to the lowest level since 2006.